Hydroelectric and wind energy are two major sources of so-called renewable energy. In the U.S.A. in 2015 (EIA), 33.3% or one-third of all electric energy is produced by steam generation using coal. A third source of renewable energy comes from the sun (only 0.6%) and a first source comes from water (hydro amounts to 6.0% according to the EIA). Water flows at variable speed and so does wind. An advantage of water flow is the mass/density, inertia or power that may be generated by the flow of water compared with the flow of wind (wind amounts to 4.7%) where wind must be collected by large wind-driven propellers or rotor blades.
Consequently, the hope of electrical energy generation for the future is in so-called renewables which include, but are not limited to, the air (wind power), the sun (solar power) and water (hydroelectric and marine hydrokinetic, MHK, energy) sources. Nevertheless, there remains a need for a wind or water driven electricity generator that may save the cost of building a dam or a large wind mill with giant propellers, permit the marine hydrokinetic (MHK) generation of electricity and use the high inertia flow of a river or the flow of ocean currents, tides and waves. Similarly, wind-driven turbines should be more efficient, reliable, and designed to convert variable wind speed over a greater speed range to constant frequency and voltage output for delivery to an electric power grid.
Further detail of a conventional wind turbine is described in WO 1992/14298 published Aug. 20, 1992 and assigned to U.S. Windpower, Inc. A variable speed rotor may turn a gearbox to increase the rotational velocity output of the rotor and blade assembly. Typically, wind speeds over 3 meters/sec are required to cause the large rotor blades to turn at the cut-in speed (rotational velocity). Wind frequency between cut-in and cut-out speeds (velocities) has been measured to vary depending on location, weather patterns and the like. Placement high on a hill or a mountain of a wind turbine, for example, may be preferable to locating the wind turbine at a low point in a valley. Consequently, it may be recognized that there are periods of time when wind turbines do not have sufficient wind speed to operate at all depending on weather conditions, placement and the like. A problem with known systems is that there is a collection of energy at variable input (variable alternating current). The variable alternating current is typically converted to direct current and then the direct current is converted to constant AC voltage and frequency alternating current. Such systems are seen by way of example in FIG. 1A (prior art).
Michael Faraday (1791-1867) is credited with the formulation of Faraday's law and at least the construction of one of the first, if not the first, direct current generator. Faraday's law may be simply stated as follows: an electromotive force may be generated in an electrical conductor (such as a copper wire or coil of wire) which encircles or is encircled by a magnetic flux, for example, caused by the presence of a permanent magnet proximate the coil or coils. Many renewable energy efforts such as the wind turbine discussed above attempt to harvest natural sources of mechanical energy (wind, tides, waves, water flow and so on) to produce electricity. Because these sources fluctuate in power such as wind energy applied, standard generators using permanent magnets and fixed windings may deliver unregulated voltage and frequency, for example, as seen by a requirement in known wind energy systems to generate DC from variable AC and then recreate a constant AC voltage and frequency from the converted DC.
New direct current generator designs such as the synchronous or induction singly-fed generator, the doubly fed generator or the brushless wound-rotor doubly fed generator are seeing success in variable speed, constant frequency applications, such as wind and other renewable energy technologies. However, such systems are complicated and are prone to failure even though they show gains in efficiency over a brush-less, commutator-free system. Consequently, problems related to known wind and water turbines relate closely to the failure rate of gearboxes, generators, variable frequency converters or variable power converters and associated electronics and inefficiencies of operation.
A solution to the identified problems is to provide a constant rotational velocity as an input to the constant speed electric generator so that the generator in turn can produce a constant frequency output and deliver a constant voltage and variable current directly to an electric grid; (a transformer may be needed). But such a solution, to the knowledge of the inventor, has not been successfully demonstrated.
A recent development in the art of gearboxes is a magnetic gear which relies on permanent magnets and avoids meshed gears. Magnetic gears, for example, developed by and available from Magnomatics, Sheffield, UK, have an air gap between sheath and shaft and so there is no meshing of gears in a mechanical gearbox to change the gear ratio of, for example, propeller or rotor input shaft to match a desired rotational velocity of an alternating current generator, for example, related to 50 Hz European or 60 Hz US. Alternating north and south poled permanent magnets are spaced side-by-side radially around a shaft and mesh with a similarly constructed magnetic gear when the permanent magnets are proximate one another. Magnetic gear sets are intended to slip with a gust of wind or burst of water energy in a magnetic gear system and then stabilize at the higher wind or water velocity. These same gears if mechanical may break gear teeth of a meshed mechanical gear gearbox.
Many of the problems of wind turbines are carried forward into marine hydrokinetic (MHK) turbines such as run-of-the-river, tidal, ocean wave and hydrokinetic river turbines. There is the same problem of having to convert a harnessed variable speed/frequency to a constant frequency and voltage output. On the other hand, for example, there are many advantages for harnessing marine hydrokinetic (MHK) energy compared with wind renewable energy: the density (mass or inertia) of water is much greater than that of wind and its speed is not as variable as wind speed especially when used in a relatively constant flowing river or stream or current which flows continuously in the same direction (such as the Mississippi River of the United States). Generally, for example, rivers flow in one direction and the major ocean currents do the same. Wave generation in oceans and other large bodies of water varies in magnitude with wind and weather. Ocean shore waves and waves caused during a storm on the ocean vary and a strong undertow or tall wave can be useful for electric power generation. Tides are reversible (high tides flowing in and low tides flowing out of a tidal estuary) and associated known turbines may be limited to one direction of water flow (high or low tide).
A concept for improving wind turbines is use of a direct drive in which a rotor and a shaft drive a generator. Such a direct drive may be used to directly drive an electric generator without using a gearbox, i.e. directly driving the generator. The failure and efficiency problems of gearboxes may be eliminated by eliminating the gearbox with direct drive. One may increase the number of poles by fifty times, for example, use power converters or frequency converters and so result in reduced down time for gearbox repairs at the expense of increased cost due to the bigger generators. A speed converter to convert variable speed to constant speed is disclosed in priority U.S. Pat. No. 8,388,481 of Kyung Soo Han. The speed converter is entirely mechanical and so scalable and improves upon the high failure rate, reliability and efficiency of known electrical/mechanical systems. Speed converters under development are also frequency converters and are shown in this and other priority patent applications and are referred to as infinitely variable speed converters or simply speed converters. The Hummingbird mechanical rotary frequency converter described in this and a priority application is a preferred speed converter to constant frequency.
Devices are also known for harnessing the power in water waves such as ocean waves. Such a device is known and available from Pelamis Wave Power. FIG. 1 of Pelamis's U.S. Pub. Patent Application 2013/0239562 of Sep. 19, 2013 shows a Pelamis device 10 floating in the ocean. The device 10 may comprise a plurality of hinged sections 12-A, 12-B, 12-C, 12-D and 12E. The device wiggles and generates power in the direction of a wave from left to right. As the wave passes through the hinged sections, the sections 12A through 12E move up and down with the height of the wave. The wave thus creates movement which may be used to generate electricity. It may be said that the higher the wave, the greater the movement; the calmer the seas, the less the movement and the less generation of electricity.
A variable torque generator (VTG) (called a VPG when varying power output) has been described in priority U.S. Pat. Nos. 8,338,481; 8,485,933; and 8,702,552 as well as priority PCT/US2010/042519 published as WO2011/011358. The variable torque generator has one of an axially moveable rotor and/or stator with respect to its stationary or moveable counterpart stator or rotor so as to vary the amount of overlap by the stator with respect to the rotor from a minimum when the stator is displaced from the rotor to a maximum value when the stator and rotor are proximate to or overlap one another. When used in a power generator to regulate flow of power, the VTG is referred to as a variable power generator or VPG. When used in a torque generator and a power generator to regulate torque and flow of power, the generator is referred to as a variable torque and power generator or VT&PG. Torque and/or power are at a maximum when there is a maximum rotor/stator overlap. In this application, such a device is referred to as a variable overlap generator or VO generator (or VOG).
There remains a need in the art to provide a direct current generator that develops direct current solely using electromotive force (EMF) without commutators and brushes. Such a direct current generator assembly may generate electrical energy/power (via a variable overlap generator and Hummingbird speed converter) and such an efficient direct current generator from renewable sources such as wind and river/tide/ocean wave/ocean current, that is, a marine hydrokinetic or wind turbine electric power generator among other possible applications for generating electric power at constant alternating current frequency and voltage for an electric power grid for a small community (for example, in developing countries) or small industrial plant (for example, 25 kw capacity) or for powering the entire Mississippi river basin (several MHK turbines placed periodically along the length of the entire Mississippi river).